Well known is the phenomenon leading to the creation of surface layers of metallic Pb (or of its salts), by segregation of Pb from the base alloy as a consequence of the thermal-mechanical stresses caused by reachining and or molding of brass alloy elements containing lead. Such a phenomenon is a particularly undesirable one, in that the creation of the said lead surface layer may easily cause, at work, the release into the environment of Pb ions, a heavy metal known to be highly polluting and toxic to human health.
On the other hand, plumbing components such as mechanic parts for cocks and valves designed to operate in potable water distribution circuits and systems, cannot but undergo, during the manufacturing process, a number of machine work operations (lathing, drilling, threading, etc.). Moreover, a Cu--Zn base alloy containing also limited amounts of Pb (generally up to 3-5% by weight) facilitates machine working and leads to more effective and accurate surface finish. Furthermore, besides facilitating machine working (it furthers chip-breaking), the presence of Pb is also instrumental to the elements forming process, whether the latter is carried out directly by smeking or by molding/die-casting.
The mechanism of Pb release has long been investigated and is based on the creation, on a zinc oxide surface layer, segregated from the base alloy, of Pb salts (hydroxycarbonates), due to surface stresses of the alloy as a consequence of both machining and shear stress during the molding process, and due further to Pb reactivity with water vapor and atmosphere carbon dioxide. It is however only very recently (March 1995) that a Certified testing procedure for evaluating the Pb release of plumbing components designed to potable water distribution has been approved and issued in print by the major United States Normalization Agency, i.e. N.S.F. The test procedure is known as U.S. NSF
STD61. It has been shown that the phenomenon of Pb release is largely present in the commercial components for potable water distribution of any type, even in those components wherein surface coating, for example chromium or nickel plating, is extensively carried out, for haestetical reasons, on all the surfaces in view: in fact, the phenomenon depends on those limited surfaces designed to remain in contact with water when the taps, cocks etc. are closed, which are internal surfaces not in view and, therefore, normally not coated and, anyway, very difficult to be coated properly.